As is known in the art, Diffusion weighted imaging (DWI) is a method used in magnetic resonance imaging (MRI) and offers a means to evaluate an area of anatomy in terms of the motion of water molecules. The level of water diffusivity can provide an indication of the structure of the tissue at the cellular level. This in turn can provide an indication of the status of the tissue. In terms of tumors within an organ such as, for example, the liver, the water diffusivity within the tumor will be less than that of the healthy tissue of the organ. This is because the cancerous cells are denser, with more impediments to water motion. In a tumor which has undergone treatment to kill off the cancerous cells, the necrotic tumor will show an increase in diffusivity compared to the viable tumor. This is because, in killing the cancerous cells, the membranes are broken down, allowing greater motion of water molecules. Thus, measurement of water diffusivity can serve as a surrogate marker to evaluate tumor treatment response.
In DWI, the MR pulse sequence is designed in such a way as to produce higher signal intensity in proportion to the water diffusivity. The sequence's b-value parameter, which is related to the MR magnetic field gradient amplitude, is a key factor in controlling the diffusion weighting. The higher the b-value, the stronger the diffusion weighting. Conversely, the signal intensity falls in an exponential manner as a function of b-value. Therefore, the higher the b-value, the lower the signal intensity, and the higher the noise. Thus, there is a tradeoff in choosing the b-value such that it gives a strong diffusion weighting, but yet still has a high signal to noise ratio (SNR).
For a certain set of tissue types, the water diffusivity difference between the tissues will be most visible using a certain b-value. While there are some common guidelines as to what b-values should be used for certain tissue types, the results will vary by patient and other scan parameters and conditions. In general, the optimum b-value is considered to be that at which the maximum contrast-to-noise ratio is achieved. Since the optimum b-value for a certain patient, anatomy and scan is not known, the level of diffusivity is usually reported in terms of the apparent diffusion coefficient (ADC), which describes the rate of signal decay as a function of b-value and can be calculated from two or more images with different b-values (considering the slope of the line described by ln(SI(b)/SI(bo)) where bo reflects no diffusion weighting); where SI is signal intensity and In is the natural logarithm. In this way it is not necessary that any single scan is using the optimum b-value. By performing the ADC calculation on a pixel by pixel basis the results are presented as a parametric image often referred to as an ADC map.
However, depending on the b-values used, and the uncertainty in signal intensity in each image at a particular b-value, the resulting ADC values will have some degree of uncertainty, which decreases their diagnostic value, particularly in terms of doing follow-up studies where a comparison is being made between ADC values pre and post treatment.
In DWI studies usually multiple slices are being acquired for each dataset. For many cases, it is also desirable that the complete dataset be captured during one breath-hold to avoid motion of the anatomy and the need for registration within a single dataset. For this reason, and because each scan takes a fair amount of time, current practice is to make the best judgment in terms of what b-values to use to obtain the best ADC maps, and keep the number of scans with different b-values to a minimum.